Normal optical fibers are uniform along their lengths. A slice from any one point of the fiber looks like a slice taken from anywhere else on the fiber, disregarding tiny imperfections. However, it is possible to make fibers in which the refractive index varies regularly along their length. These fibers are called fiber gratings because they interact with light like diffraction gratings. Their effects on light passing through them depend very strongly on the wavelength of the light.
A diffraction grating is a row of fine parallel lines, usually on a reflective surface. Light waves bounce off of the lines at an angle that depends on their wavelength, so light reflected from a diffraction grating spreads out in a spectrum. In fiber gratings, the lines are not grooves etched on the surface, instead they are variations in the refractive index of the fiber material. The variations scatter light by what is called the Bragg effect, hence fiber Bragg gratings (FBGs). Bragg effect scattering is not exactly the same as diffraction scattering, but the overall effect is similar. Bragg scattering reflects certain wavelengths of light that resonate with the grating spacing while transmitting other light.
FBGs are used to compensate for chromatic dispersion in an optical fiber. Dispersion is the spreading out of light pulses as they travel on the fiber. Dispersion occurs because the speed of light through the fiber depends on its wavelength, polarization, and propagation mode. The differences are slight, but accumulate with distance. Thus, the longer the fiber, the more dispersion. Dispersion can limit the distance a signal can travel through the optical fiber because dispersion cumulatively blurs the signal. After a certain point, the signal has become so blurred that it is unintelligible. The FBGs compensate for chromatic (wavelength) dispersion by serving as a selective delay line. The FBG delays the wavelengths that travel fastest through the fiber until the slower wavelengths catch up. The spacing of the grating is chirped, changing along its length, so that different wavelengths are reflected at different points along the fiber. These points correspond to the amount of delay that the particular wavelengths need to have so that dispersion is compensated. Suppose that the fiber induces dispersion such that a longer wavelength travels faster than a shorter wavelength. Thus, a longer wavelength would have to travel farther into the FBG before being reflected back. A shorter wavelength would travel less far into the FBG. Consequently, the longer and shorter wavelengths can be made coincidental, and thus without dispersion. FBGs are discussed further in Feng et al. U.S. Pat. No. 5,982,963, which is hereby incorporated herein by reference in its entirety. A circulator is used to move the reflected beam onto an different path from the input beam.
FBGs are typically fabricated in two manners. The first manner uses a phase mask. The phase mask is quartz slab that is patterned with a grating. The mask is placed in close proximity with the fiber, and ultraviolet light, usually from an ultraviolet laser, is shined through the mask and onto the fiber. As the light passes through the mask, the light is primarily diffracted into two directions, which then forms an interference pattern on the fiber. The interference pattern comprises regions of high and low intensity light. The high intensity light causes a change in the index of refraction of that region of the fiber. Since the regions of high and low intensity light are alternating, a FBG is formed in the fiber. See also Kashyap, xe2x80x9cFiber Bragg Gratingsxe2x80x9d, Academic Press (1999), ISBN 0-12-400560-8, which is hereby incorporated herein by reference in its entirety.
The second manner is known as the direct write FBG formation. In this manner two ultraviolet beams are impinged onto the fiber, in such a manner that they interfere with each other and form an interference pattern on the fiber. At this point, the FBG is formed in the same way as the phase mask manner. One of the fiber and the writing system is moved with respect to the other such that FBG is scanned or written into the fiber. Note that the two beams are typically formed from a single source beam by passing the beam through a beam separator, e.g. a beamsplitter or a grating. Also, the two beams are typically controlled in some manner so as to allow control over the locations of the high and low intensity regions. For example, Laming et al., WO 99/22256, which is hereby incorporated herein by reference in its entirety, teaches that the beam separator and part of the focusing system is moveable to alter the angle of convergence of the beams, which in turn alters the fringe pitch on the fiber. Another example is provided by Glenn, U.S. Pat. No. 5,388,173, and Stepanov et al., WO 99/63371, which are hereby incorporated herein by reference in their entirety, and teach the use of an electro-optic module, which operates on the beams to impart a phase delay between the beams, which in turn controls the positions of the high and low intensity regions.
Each manner has advantages and disadvantages when compared with each other. For example, the first manner, the phase mask manner, is relatively inflexible, as changes cannot be made to the mask. However, since the phase mask is permanent, the phase mask manner is stable, repeatable, and aside from the cost of the mask, relatively inexpensive to operate. On the other hand, the direct write manner is very flexible, and can write different gratings. However, this manner is less repeatable and is costly to operate.
Another problem with the phase mask manner resides in the fabrication of the masks. Masks are fabricated by lithographic or holographic techniques. The mask slab is coated on its surface with light or particle (electron or ion) sensitive or photosensitive material (resist). Under the resist, the slab may also be coated with a metallic layer (e.g. chrome) to assist conduction of charged particles away from the exposed regions. Regions or bars of the resist are illuminated by light or particle beam according to a desired pattern, which is generally an array of parallel bars along a straight line with precisely selected positions. This illumination causes chemical changes in the exposed regions of resist. The exposed resist can be preferentially removed from the slab by a chemical or plasma, which does not strongly affect the unexposed resist (or vice versa). After the preferential removal of the resist according to the desired pattern, the slab may then be etched by a different chemical or plasma which preferentially etches the slab where the resist has been removed. The etched portions of the slab have a difference in thickness or height from the un-etched portions. When the etched (bars) and un-etched (spaces) portions are patterned to form an array along a straight line, the differences in thicknesses form a phase grating. Thus, by applying an array of bars and spaces on the slab to form a grating, a phase mask will be formed. Other lithography tools can directly etch the bars and spaces onto the mask rather than in resist. In another embodiment, these regions can have alternate transmittance properties, such as by the presence or absence of an opaque material (e.g. chrome), and thus form an amplitude grating. Note that in all these cases, the critical part of the fabrication is the exposure of the bars and spaces (or direct etching of the bars and spaces). The resulting mask is limited by the quality and precision of the exposure process.
Current lithographic techniques used in the fabrication of phase masks have a limitation referred to as xe2x80x98stitching errorxe2x80x99. This originates from repositioning and re-magnification errors, which have the effect of slightly misplacing the periodic structure required in the mask. New optical and e-beam lithography tools have the capability to write continuous patterns (so called cursive writing) effectively without such stitching errors. However, the currently practiced state of the art cannot utilize cursive writing to make masks for chirped FBGs and/or FBGs with phase shifts (positional shifts of the bars or spaces, or changes in the bar or space widths), without the introduction of stitching errors.
These and other objects, features, and technical advantages are achieved by a system and method system which uses current lithography tools to fabricate masks without stitching errors from re-scaling or re-positioning. The masks fabricated by the invention will generate the linear or non-linear chirp, and other phase shifts as desired, in the fiber Bragg grating (FBG) in the core of the fiber.
The invention preferably uses the pixelation of the resist exposure or direct etching. The invention preferably xe2x80x98feathersxe2x80x99 the pixels of the mask lines by adding, removing, and/or displacing one or more pixels. Thus, the bars of the mask are not smooth and continuous (at the pixel resolution), but rather have pixels added, removed, and/or displaced at the edges of the bars. This addition, removal, and/or displacement of pixels will affect the FBG being written into the fiber. Since the fiber is operating single mode, any variations in the location of the written index modulation, which is transverse to the fiber axis, will be averaged out over the core diameter. In other words, a single position is defined which is the effective location of each periodic variation (edge) of the index modulation in the core. This allows the achievement of much finer resolution FBGs than the pixel size of the mask, because of the averaging effect that occurs with the feathering of the edges of the mask bars.
Thus, the invention preferably achieves a resolution that is 1/N of one pixel, where N is the number of pixels used to form a bar in a direction that is transverse from the fiber axis. This fine resolution allows fine details to be encoded in the FBG that is formed in the fiber. Such details could include linear and non-linear chirps in the pitch of the grating, and arbitrary discrete or continuous phase variations. That is, the position of the bars and spaces can be positioned according to any desired pattern with resolution improved by a large factor over conventional methods, and without the introduction of stitching errors.
The invention also uses a focusing lens system, which is used to focus the light through the mask and onto the fiber. This allows a larger portion of the mask to be illuminated by the light source; larger in the direction that is transverse from the fiber axis. This, in turn, allows for more illuminated pixels on a given bar to be focused and thus averaged in the formation of the grating within the fiber, which provides even greater resolution. That is the larger number of pixels, N, which are effectively illuminated and focused onto the core of the fiber, the better the possible resolution. Since the focusing need only be in the direction orthogonal to the fiber axis, it is preferable to have the focusing lens system comprise at least one cylindrical lens.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.